Impact copolymers (ICP's) are commonly used in a variety of applications where strength and impact resistance are desired, such as molded and extruded automobile parts, household appliances, luggage, and furniture. Propylene based ICP's are typically an intimate mixture of a continuous phase of a crystalline polypropylene polymer and a dispersed rubbery phase of a secondary polymer, e.g., an ethylene copolymer. While these so-called ICP products have been produced by melt compounding the individual polymer components, multi-reactor technology makes it possible to produce them directly. This is accomplished by polymerizing propylene in a first reactor and transferring the polypropylene polymer from the first reactor into a second reactor where the secondary copolymer is produced in the presence of the polypropylene polymer.
A variety of polymerization processes can be used to prepare the crystalline polypropylene polymer and the secondary polymer, such as gas phase, slurry, liquid, and/or solution polymerization. It is also quite common to make the constituent polymers in two different polymerization processes, for example, slurry phase for the polymerization of the polypropylene polymer and gas phase for the polymerization of the ethylene copolymer. The composition, composition distribution, amount, molecular weight, and/or molecular weight distribution of the secondary polymer primarily determines the engineering properties of the ICP. Accordingly, control over the secondary polymer is desirable in order to control the properties of the ICP produced via the two stage polymerization process.
One problem encountered in a two stage or two step polymerization process that uses gas phase polymerization for the secondary ethylene copolymer is that the catalyst, e.g., titanium catalyst, tends to degrade in activity and/or selectivity toward the polymerization of the secondary polymer. As such, the amount of the secondary polymer made in the second polymerization stage is typically less than desired. Additionally, degradation of the catalyst's selectivity toward a particular copolymer causes the secondary copolymer to have a range of molecular weights, varying composition, and/or other varying properties that cannot be controlled.
There is a need, therefore, for improved polymerization processes for making ICP's having improved catalyst activity and/or selectivity toward the polymerization of the secondary polymer.